This invention relates to a method and apparatus for pyrometrically measuring temperatures of an object within a range of between about 100.degree. C and about 2700.degree. C.
There is a problem in pyrometrically measuring the temperature of an object having a large range of temperatures such as, for example, between 100.degree. C and 2700.degree. C. The object may be, for example, in the form of a graphite tube atomizer as used in flameless atomic absorption spectroscopy. In such atomizers, the graphite tube is heated by passing an electric current therethrough. A sample is inserted into this graphite tube and, during the heating process is first dried, then ashed, i.e., decomposed chemically, and eventually atomized at a high temperature. A beam of radiation of the resonance spectral line of the element to be detected is then passed through the atomic cloud to a suitable detector. The absorption, to which this beam of radiation is subjected, in the atomic cloud, is a measure of the proportion of the element being detected in the sample. During atomization, it is necessary to heat the sample to the desired atomization temperature as quickly as possible, while the slope of the temperature increase should be independent of the final temperature value.
In a prior art apparatus of this type, as described in "Analytical Chemistry", Vol. 46 (1974), No. 8, pages 1028-1030, the heating was effected with full heating power, until a preselected desired temperature was reached. This desired temperature was automatically maintained by switching the heating power on and off. In the prior art apparatus, temperature measurement was effected photometrically. To this end, radiation from the graphite tube was directed to a photodiode. A red filter was placed in front of the photodiode, which cut-off the wavelength range below 620 nm. By the elimination of the short-wave radiation, ambiguity of the output signal was avoided in a portion of the temperature range, said ambiguity resulting from the combination of the sensitivity characteristics of the diode and the variation of the wavelengths of the radiation, as a function of temperature. In such graphite tube atomizers, a rather large temperature range has to be covered, which extends from a relatively low drying temperature to the very high temperatures, which are required to atomize substances that are difficult to volatize. In the prior art apparatus, a temperature range of between 550.degree. C and 2600.degree. C was covered. This resulted in a variation of the photocurrent through a plurality of orders of magnitude, due to the fact that the spectral sensitivity of the photodiode combined with the filter was located on the short-wave slope of the spectral intensity distribution of the radiation source. Therefore, with increasing temperature, the spectral intensity distribution characteristics were shifted towards the range of sensitivity of the photodiode, in accordance with Wien's displacement law. As a result, the portion of the total radiation, which could be detected by the photodiode, increased with increasing temperature of the object. In addition, an increase in temperature involved a large increase in the radiation intensity. Variation of the output signal through a plurality of orders of magnitude within the temperature range to be covered presents extraordinary problems in the signal processing and in the dimensioning of the temperature control loop. The lower limit of the measuring range for the temperature was determined by the spectral sensitivity of the photodiode. If the photodiode was sensitive between 62 and 2 microns, only a very low proportion of the total radiation impinged upon the photodiode, when the temperature of the object was 100.degree. C. Only at a substantially higher temperature would this proportion become measurable. With the means described in the above-described prior publication, it was, in fact, not possible to cover the desired measuring range of between 100.degree. C and 2700.degree. C.
Furthermore, it is known to use the radiation in a limited wavelength range for pyrometric temperature measurements. In the prior art methods, the purpose of the limitation of the wavelength range was to avoid disturbances due to absorption bands of carbon dioxide or water vapor. In a prior art method of this type (German Pat. No. 2,013,723), the temperatures of the textile surfaces in textile steaming plants were measured. Wavelength ranges between 3.4 and 4.5 microns or 9 to 10.5 microns were used. In this application, the measurement took place within a closely limited temperature range, the temperatures being of the order of 100.degree. C.
Another prior art method relates to the measurement of the temperature from the radiation of refractory stones, bricks, or similar material. Also, in this method, disturbance of the measurement by the absorption of carbon dioxide or water vapor should be avoided, and for this purpose, a band filter, having a spectral transmission range of between 7.2 and 8.2 microns, was used. Also, in this prior art method, the temperature range in which the actual measurement took place was closely limited, as described in German Pat. No. 2,214,722.
It will be appreciated that both of the prior art methods just described measured the temperature over a very limited temperature range.